Background information can be found in textbooks and places like the CDC where information has been compiled on a particular topic.
Access Medicine contains many core texts such as Hurst's The Heart, Harrison's Internal Medicine or Ganong's Review of Medical Physiology.
From the National Library of Medicine this resource contains overviews of health conditions and links to published literature in PubMed.
Many diseases and conditions have dedicated associations or organizations that focus on research and education. Below are a few examples.